1. Field of the Invention
This invention relates generally to internal combustion engines of the kind having a reciprocating piston whose reciprocating motion produces cyclic pressure fluctuations in the engine crankcase. The invention relates more particularly to a novel fuel pump for such engines.
2. Discussion of the Prior Art
As will become readily evident from the following description, the fuel pump of the invention may be adapted for use on a variety of engines and engine sizes. One particularly useful application of the fuel pump, however, is on a small internal combustion engine of the kind used in radio controlled model airplanes and the like. The fuel pump will be described in the context of this particular application.
A typical model airplane engine is a single cylinder two-cycle engine having a body providing a cylinder proper and a crankcase. The crankcase chamber opens through the front and rear ends of the crankcase, and these open ends are closed by front and rear end plates which are bolted to the crankcase. Journelled in the front end plate in a crankshaft having an inner crank end within the chamber connected by a connecting rod to a piston movable in the cylinder and an outer coupling end which extends externally of the engine for mounting a propeller. The rear crankcase end plate of the standard engine serves merely as a closure for the rear end of the crankcase.
Fuel and air enter the engine through a carburetor having an intake air venturi and a fuel jet which opens to the venturi throat. The venturi communicates to the engine crankcase chamber through a valve which opens and closes in timed relation to the reciprocating motion of the piston in such a way that air is drawn into the chamber during each upstroke of the piston and then displaced from the chamber into the cylinder during the following downstroke of the piston. The resulting air flow through the venturi produces in the venturi throat a partial vacuum which draws fuel into the intake air stream from the engine fuel tank through the fuel jet. The carburetor has a throttle valve which is adjustable to regulate the air and fuel flow to the engine and thereby engine speed.
This method of utilizing carburetor venturi vacuum to induce fuel flow to the engine has several disadvantages which are well known to those skilled in the art of model airplanes and hence need not be elaborated on. Suffice it to say that the rate of fuel flow to the engine is extremely sensitive to fuel tank conditions, fuel head pressure, and the forces of flight active on the fuel, particularly at idling and low speed settings of the throttle valve, with the result that the engine often runs excessively lean or excessively rich and frequently cuts out. Moreover, since the engine lubricating oil is contained in the fuel, the delivery of an excessively lean mixture to the engine may result in serious engine damage.
In an effort to avoid the above and other disadvantages of the simple vacuum draw fuel system discussed above, a variety of pressurized fuel systems have been devised. Most of these pressurized fuel systems have utilized in one way or another the pressure fluctuations which occur in the crankcase of single cylinder two-cycle engines, such as model airplane engines, during engine operation. In this regard, it is well known that during operation of such an engine, each upstroke of the piston produces a partial vacuum in the crankcase through a then open valve. During the following downstroke of the piston, the valve closes and the mixture is displaced from the crankcase into the engine cylinder, thereby producing a positive pressure in the crankcase. Thus, during engine operation, alternate negative and positive pressure pulses are produced in the crankcase.
Attempts have been made to utilize these pressure pulses to effect a fuel pumping action or othewise achieve pressurized fuel delivery. For example, pulse pressure pumps operable by the crankcase pressure fluctuations have been tried. However, these pumps were not successful for the reason that they were connected to the engine crankcase by tubes or the like which damped out the pressure fluctuations at the pump above a certain engine speed. Other pressurized fuel systems utilizing the crankcase pressure pulses are not totally successful or satisfactory for model airplane use for the reason that they lack fuel pressure regulating means, which are required to obtain a relatively constant fuel pressure over the total engine operating range or utilize fuel pressure regulating arrangements that are ill-suited to model airplane use because of their size and complexity or their reliance on vacuum to draw fuel through the carburetor.
By way of example, one of the most commonly used pressurized fuel systems for model airplane engines is the so-called "timed crankcase pressure system". In this system, the fuel tank is connected to the engine crankcase through a fitting which allows only positive pressure pulses to the tank. As a result, the tank pressure may build up to several pounds when the engine is running at high speeds. When the engine is throttled back, this high tank pressure will cause flooding of the engine. On the other hand, if the idle setting of the fuel system is made to accommodate the high pressure build up during high speed engine operation, low speed engine operation with the fuel tank depressurized will result in the delivery and an excessively lean mixture to the engine, thereby creating the possibility of engine damage or cessation of engine operation.
An improved system of this latter kind has been devised with a pressure regulating feature to avoid the problems discussed. This improved system is still sensitive to fuel head pressure, is relatively complex, and requires substantial modification of the engine and hence is not totally satisfactory.